1. Field of the Invention
The present invention relates to an electrostatic capacitive touch sensor device for use in a switch device for operational input to, for example, an electronic device.
2. Description of the Related Art
Conventionally, in order to operate, for example, an audio device mounted in a car, a switch panel device is installed at a position on the dashboard allowing easy operation by the driver or passenger. Several switch panel devices as described are provided with electrostatic sensors.
In a switch panel device of this type, a sensing electrode is formed at a portion of a panel section which an operator touches to operate the switch panel device. The sensing electrode is applied with a signal (voltage) output from a power supply. The sensing electrode is connected to a sensing circuit.
When an operator touches the sensing electrode, a slight amount of current flows from the sensing electrode to the operator. Therefore, a voltage value changes in a download side of the sensing electrode between when an operator touches the sensing electrode and when the operator does not touch the sensing electrode. By detecting the change, the sensing circuit senses that the switch panel device has been operated.
The sensing circuit is provided outside the panel section of the switch panel device. Therefore, a connection pattern for electrically connecting the sensing electrode to the sensing circuit is formed on the panel section.
However, when an operator touches the connection pattern through the panel section, a slight amount of current then flows to the operator, and a voltage value in a download side of the connection pattern therefore changes. Even in this case, the sensing circuit detects change of the voltage value. As a result of this, the operator is recognized to have touched the sensing electrode. It is not preferred that the sensing circuit detects change in voltage which is caused by a touch of the operator on the connection pattern.
Therefore, a shield electrode for covering the connection pattern is provided so that the sensing circuit may not detect a touch when an operator touches a connection pattern. Even when an operator touches a portion where the connection pattern is provided in the panel section, a current flows from the shield electrode to the operator while a current is restricted from flowing from the connection pattern to the operator.
According to the prior art for a shield method of this type, the shield electrode is branched from between the sensing electrode and the sensing circuit. A technique of this type is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-268607.
Therefore, in the case of using a plurality of sensing electrodes to form a large number of operation switches like an operation switch for a car, a plurality of independent shield electrodes respectively paired with the sensing electrodes, and a plurality of independent circuit means are required between the shield electrodes and the sensing electrodes are required. Consequently, if a large number of operation switches are provided, not only the shield electrodes are difficult to be laid out but also costs for the circuit means for supplying signals to the shield electrodes increase.
Further, in a car occupant sensing system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-268607, a shield electrode is branched from between a sensing electrode and a sensing circuit. If electrical noise which propagates in the air is applied to the sensing electrode, a signal supplied to the shield electrode becomes an electrical signal equal to the noise, and the effect of shield against electrical noise decreases.
That is, on a wiring connecting the sensing circuit and the sensing electrode, a voltage acting on the wiring changes owing to application of noise to the sensing electrode. Therefore, in the case of a structure in which a shield electrode is branched from a wiring connecting a sensing circuit and a sensing electrode, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-268607, a voltage value applied to the shield electrode is influenced by change in voltage caused by noise affecting the sensing electrode.
As a result of this, the voltage value applied to the shield electrode is considered to change under influence of external noise.